Lithium batteries have become established as energy stores above all for applications in portable electronics (laptops, mobile telephones), because of their high energy density and power density in comparison to other battery types. A distinction is made between primary lithium batteries, which are non-rechargeable batteries having mostly lithium metal anodes, and secondary systems, in other words rechargeable batteries.
Both battery types contain anhydrous liquid or gel-like ion-conductive electrolytes, in which supporting electrolytes, for example LiPF6, LiBF4, lithium imides, lithium methides or lithium borate salts, for example lithium bis(oxalato)borate (LiBOB, corresponding to Li[B(C2O4)2]), are present in dissolved form.
In comparison to lithium element fluorides such as LiPF6 or LiBF4, lithium borate salts such as LiBOB bring about a significant improvement in cycle stability and safety properties in secondary lithium batteries (Cox, S. S. Zhang, U. Lee, J. L. Allen, T. R. Jow, J. Power Sources 46, 2005, 79-85). This is due to a modified form of protective coating formation on the carbon anode of a lithium battery: borate electrolytes give rise to the formation of a thin, very stable Li+-conductive coating on this anode, which is stable even at elevated temperatures and thus prevents dangerous decomposition reactions between the charged anode and the electrolyte, for example (J.-C. Panitz, U. Wietelmann, M. Wachtler, S. Strobele, M. Wohlfahrt-Mehrens, J. Power Sources 153, 2006, 396-401; Chemetall brochure 2005). The improvements to the protective coating brought about by borate salts offer users new possibilities for electrolyte formulation.
For instance, the difficult-to-handle ethylene carbonate(1,3-dioxolan-2-one), for example, can be abandoned in favour of propylene carbonate(4-methyl-1,3-dioxolan-2-one) (K. Xu, S. Zhang, R. Jow, J. Power Sources 143, 2005, 197-202). It is also possible, moreover, to dispense with 1,3-dioxolan-2-one compounds altogether and instead to use γ-lactones, for example γ-butyrolactone (US-A-2007/0065727).
DE-C-19829030 discloses a number of methods for producing LiBOB:    1. Reaction of lithium boron hydride with anhydrous oxalic acid:
                A disadvantage in addition to the high cost of LiBH4 is a secondary reaction in which oxalic acid or the oxalate anion is attacked and reduced by the hydride.            2. Reaction of lithium hydroxide or lithium carbonate with boric acid or boron oxide and oxalic acid in aqueous solution and subsequent product drying, for example:
                Variants of this reaction involve reacting two of the three raw material components in advance and only then carrying out the LiBOB synthesis, in other words for example:        
                Other suitable raw materials are LiHC2O4 or LiBO2.            3. Reaction of the raw materials cited in 2. in an organic solvent, for example toluene, and removal of the water formed by means of azeotropic distillation.    4. Reaction of lithium alkoxides and boric acid esters with anhydrous oxalic acid in a solvent, for example an alcohol:
                Finally, performing the reaction described in (2) without addition of water or other solvent in the heterogeneous phase is known from DE-C-10108608.        
Common to all of the cited processes is that the LiBOB is not produced in a sufficiently pure form. It is contaminated with varying amounts of water, acid components and insoluble by-products, for example lithium oxalate (Li2C2O4) or lithium carbonate (Li2CO3). When crude LiBOB salt is dissolved in aprotic solvents such as esters or nitriles, extremely turbid solutions form as a consequence. The insoluble proportion is typically between 0.5 and 2 wt. %, and homogenised solutions exhibit turbidities of more than 100 NTU (NTU=nephelometric turbidity unit), typically of 200 to 1000 NTU.
For that reason the crude LiBOB salt has to undergo a purification process. According to the prior art this consists of a recrystallisation from acetonitrile (AN). To this end a saturated, clear LiBOB solution in acetonitrile is first produced and then toluene is added. The toluene expels LiBOB from the solution and a needle-shaped crystallisate is formed, consisting of a LiBOB•AN complex with AN as solvate. This complex is then vacuum-dried, for example at 80° C. over several days (W. Xu, C. A. Angell, Electrochem. Solid-State Lett. 4 (2001), E1-E4). The bonded AN is removed in this drying procedure, destroying the crystal form. The largely solvate-free LiBOB formed in this way is obtained in a form as fine as dust, which is extremely difficult to handle. In a similar way LiBOB crystallises out of many other solvents, for example tetrahydrofuran (THF) or ethyl acetate, in solvated form too. As all the solvents mentioned are unconventional or undesirable in batteries, they have to be completely removed before use. As in the case of AN, this produces fine, hygroscopic powders which can be handled only with great difficulty.
In addition it is difficult to remove the last residues of solvent completely. It is known that ethyl acetate, even in relatively small concentrations, can adversely affect the high-temperature resistance of lithium-ion batteries (T. R. Jow, K. Xu, M. S. Ding, S. S. Zhang, J. L. Allen, K. Amine, J. Electrochem. Soc. 151, A1702-A1706 (2004)).